Electrically converted fir tube boiler and method

ABSTRACT

A modified fire tube boiler construction and method enables a conventional fossil fuel fired fire tube boiler to operate with electrical heating for the purpose of heating water and producing low and high pressure steam. The construction and method involve replacing a plurality of selected fire tubes with a like number of elongated electrical immersion heaters supported by heater support rods between the front and rear tube sheets and controlling the heaters selectively and electrically for temperature, pressure and water level sensing.

United States Patent Hoffman [54] ELECTRICALLY CONVERTED FIR TUBE BOILER AND METHOD [72] Inventor: John A. Hoffman, 4010 Camelot Dr., Raleigh, N.C. 27609 [22] Filed: July 6, 1971 [21] Appl. No.: 159,960

[52] US. Cl. "219/321, 122/4 R, 431/156 [51] Int. Cl. ..F24h 1/00 [58] Field of Search ..2l9/200, 201, 279, 321, 544, 219/546; 431/156; 122/4 R [56] References Cited UNITED STATES PATENTS 1,984,262 12/1934 Glenn.. ..219/279 3,280,297 10/1966 Folmar ..2l9/201 [4 1 Sept. 12, 1972 Primary Examiner-C. L. Albritton Attorney-B. B. Olive [57] ABSTRACT A modified fire tube boiler construction and method enables a conventional fossil fuel fired fire tube boiler to operate with electrical heating for the purpose of heating water and producing low and high pressure steam. The construction and method involve replacing a plurality of selected fire tubes with a like number of elongated electrical immersion heaters supported by heater support rods between the front and rear tube sheets and controlling the heaters selectively and electrically for temperature, pressure and water level sensing.

8 Claims, 5 Drawing Figures PATENTEDSEP 12 i972 INVENTOR John A. Hoffman BY I g zp/m ATTORNE Y ELECTRICALLY CONVERTED FIR TUBE BOILER AND METHOD BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates broadly to immersion type electrical water heaters but more specifically to apparatus and methods concerned with converting fossil fuel fired fire tube boilers to electrical heating.

2. Description of the Prior Art Steel fire tube boilers consist of a shell containing water with numerous horizontal tubes passing through the shell and fastened to tube sheets at the front and rear of the boiler. When burning fossil fuels the hot gases leave the combustion chamber, pass through the tubes thus heating the water or producing steam and then are vented through a chimney to the atmosphere. The efficiency of fossil fuel fired boilers can vary from 40 to 80 percent depending on the age and maintenance provided for the boiler. Poor combustion often causes unburned fuel to go to the atmosphere without providing any heat to the system and thus excessively polluting the air. In comparison, the efficiency of the electric heat boiler can be close to 100 percent because all the heat has to pass through the water. It is also recognized that radiation losses can be kept to a minimum with insulation.

Numerous schools, public institutions, factories and the like are equipped with fire tube boilers adapted to use fossil fuel. Since many of these installations have become surrounded over the years by residentially zoned land the emission of smoke, gases and other pollutants is not desirable. Furthermore, the boilers have in many instances become increasingly expensive to maintain and operate for fossil fuel operation. Yet in spite of the fact that the situation has been recognized for a substantial number of years there has been almost no effort directed to removing the outmoded fire tube boilers and replacing them with more modern electrical systems. Complete abandonment of a fire tube boiler obviously involves a considerable capital expense and is resisted by school districts, public institutions, hospitals and the like. Therefore, the problems associated with pollution, poor maintenance and operating costs have risen dramatically. While various prior art has been directed to converting coal fired boilers to gas fired boilers and vice versa there has been no meaningful solution to the problem of converting fossil fuel fired fire tube boilers to a non-polluting and more efficient form of heating. Specifically, the prior art has provided no way to use the basic physical structure of a fire tube boiler designed for fossil fuels and convert that structure to electrical heating.

SUMMARY OF THE INVENTION According to the invention, a conventional fire tube boiler designed for fossil fuels is converted for electrical heating. The modified boiler construction and conversion method first involves removing the fossil fuel burning equipment. The front cleanout doors and rear breeching are positioned to expose the front and rear tube sheets and tube ends. Selected fire tubes are cut out and each is replaced by an assembly which includes an immersion type electrical heater and a support rod which extends between the front and rear tube sheets. Fluid sealing arrangements are provided at each respective juncture of the heater assembly with the tube sheet. The heaters are then wired for electrical supply, selective energization and appropriate controls related to temperature, pressure and water level. Insulation is added to the combustion chamber and other areas to minimize radiation losses.

The output of a fossil fuel fired boiler is rated by furnace volume and total heating surface and the boiler therefore assumes a particular size for each rating. The electric conversion boiler, as constructed according to the invention, is however limited only by the number of electric immersion heaters that can be physically installed in the shell. An example of this can be seen by noting that by using electric elements rated at approximately 4 kilowatts per linear foot of element only onethird of the fire tubes need to be replaced with electric elements to achieve the same output rating as that achieved by fossil fuel firing. Therefore, by replacing all the fire tubes with immersion heaters, three times the rating of fossil fuel firing can be achieved with the same boiler. The immersion type electric heaters may be wired to provide a number of steps. Thus, the low water cutoffs may be wired so that as the water level drops only the elements that are closest to the surface will be cut-off enabling the boiler to continue to operate safely until the boiler is almost empty. This arrangement also reduces element burn-out.

DESCRIPTION OF THE DRAWINGS FIG. I is a somewhat schematic side elevation view of a fire tube boiler converted according to the invention.

FIG. 2 is a somewhat schematic elevation view of the boiler.

FIG. 3 is a partial elevation section view showing a typical heater assembly.

FIG. 4 is a partial plan section view of the heater assembly.

FIG. 5 is a generalized and schematic wiring diagram of a typical control and power circuit suited to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention is broadly applicable to fire tube boilers. A particular class of boiler to which the invention is applicable is the so-called steel fire tube boiler such as the High Firebox Type C, Scotch Marine Type, and HRT Type. The drawings and the present description use the conversion of a Type C High Firebox Low Pressure Boiler as an example.

As shown in the drawings the steel fire tube boiler consist of a shell 10 containing water with numerous fire tubes 11 passing through the shell 10 and fastened to a'front tube sheet 12 and a rear tube sheet 13. Fossil fuel is conventionally burned in the combustion chamber 20 and the hot gases pass through the respective front and rear smoke boxes 21, 22 and through fire tubes 11. The gases then escape through a chimney, not shown.

When used as a water boiler the water line is typically at level W and when used as a steam boiler, the water line is typically at level W To commence the conversion according to the invention the redundant fossil fuel burning equipment is removed. Next the front cleanout doors, not shown, are opened so as to expose the front tube sheet 12 and the front ends of fire tubes 11. An appropriate section of the breeching at the rear of the boiler is next removed so as to expose 'the rear tube sheet 13 and the rear ends of fire tubes 11. At this stage the ends of a selected number of fire tubes are cut out with a welding torch or other cutting device and are removed from the boiler.

For each fire tube removed there is installed an immersion heater assembly as shown in FIGS. 3 and 4 and which is next described. Each heater assembly, generally designated 25, includes on one tube end a mounting sleeve 26, a gasket 27, and a lock nut 28. The assembly further includes an immersion type electric heater 30, a support rod 31 and a plurality of electrically insulating hangars 32. Support rod 31 is supported on the opposite tube end by a hole plug assembly 29 which includes a flange 33, a gasket 34, a threaded draw bolt 35 secured to flange 33, a locking bar 36 and a tightening nut 37. A suitable bracket 40 is welded to flange 33 and provides support for support rod 31 to which bracket 40 is welded or otherwise secured. Power leads 41 connect to heater 30 which is supported in sleeve 26 by a threaded connection 45.

After the fire tubes are removed, the heater assemblies 25 are inserted in the now available fire tube openings in the front tube sheet 12 and each respective lock nut 28 is fastened which seals the front tube sheet opening and positions the heater assembly.

As the next step, each opening in the rear tube sheet 13 is closed by a plug assembly 29 by means of tightening a respective nut 27 on a respective draw bolt 35 so as to bring the respective flange 33 and locking bar 36 into a hole sealing relation, as shown in FIGS. 3 and 4.

At this stage, the respective immersion heater 30 will have been suitably supported on its respective support rod 31 by hangers 32. It should be understood in this regard that much of the work required is able to be performed by workmen standing within the boiler and who may or may not follow the exact sequence just described. Thus, the heater assemblies can be partially assembled external of the boiler and final assembly completed along with installation within the boiler.

After an appropriate number of immersion heater assemblies have been installed a suitable sheet insulation 50 is next installed around the combustion chamber and the front and rear tube sheets 12, 13, since these are no longer exposed to flames.

After the electrical heater assemblies and insulation have been installed, the final step concerns wiring the heater assemblies both for power and control purposes. Safety is of course a prime consideration in any boiler operation and the present invention lends itself readily to a safe operation. While various power and control circuit arrangements may be employed it has been found desirable to treat each horizontal row of heater assemblies as a unit. Accordingly, all heaters in the top horizontal row are connected with power wiring through a control contactor, appropriate safety devices, a time delay and then to a circuit breaker in the power panel. The control contactor may operate on either steam pressure or hot water temperature. The safety devices should include both a low water cut-off and a high pressure or temperature cut-off. The time delay insures that only one horizontal row of heaters will be energized at a time.

Each horizontal row of heaters is wired in the manner indicated and element leads are shown at 55 in FIG. 2 and should be understood as leading to the respective contactors and circuit breakers. In FIG. 5 the typical panel board circuit breaker is indicated at 60 and is shown connected to a representative heating contactor 61 to which are connected the lines 62 leading to the various immersion type electric heaters 30. Appropriate low water cut-off switches which may be wired singularly or in groups are indicated at 63 and are represented as being connected to a sequence controller 64 which may have whatever switches are required for the desired number of steps of heat. A time delay relay may be required for power demand limiting and this is indicated at 65. A pressure switch or boiler safety is indicated at 66 and is connected to the control power indicated at 67. A sequence controller activator which may be pneumatic, hydraulic, electronic or electric is indicated at 68 and is shown connected to the sequence controller 64.

For further power demand limiting purposes there is indicated a recycling relay 69 connected to sequence controller activator 68. A temperature controller is represented at 70 and this may be, for example, pneumatic, electronic or electric and is controlled by a senso'ring element 71 which may be temperature of pressure sensitive and may be electronic, pneumatic, liquid or vapor capillary type or rod and tube type. This general description is, of course, given merely to indicate one of the many possible types of control systems that may be usefully employed with the invention.

With all of the steps completed as described it can be seen that the invention method and construction provides a modified and much improved electrically heated boiler in which a selected plurality of fire tubes remain and others of which are replaced by electric immersion heaters secured between and supported by the front and rear fire tube sheets.

As previously explained, approximately one-third of the fire tubes can be replaced to achieve the same output rating as with fossil fuels. Note has also been made of the dramatic increase in boiler efficiency due to direct contact between the water and immersion heaters. Elimination of moving parts and handling of fuel also sharply reduces operating and maintenance costs. The ease of adapting the invention to safety requirements associated with pressure, temperature and water level has also been pointed out. The versatility of the invention is also found in the fact that some of the immersion heater elements may be installed in each horizontal row and treated as spares so that in the possible event of an element burn-out the spare can be wired and used without requiring a shutdown until normally scheduled. Of particular importance is the fact that such a conversion implies an immediate transfer of any air pollution problem back to the central electric generating plant which is much more suited to practical pollution control.

What is claimed is:

1. The method of converting a fossil fuel fired fire tube boiler of the type having fire tubes extending between front and rear tube sheets, an associated combustion chamber, smoke boxes and fossil fuel burning equipment, comprising the steps:

a. removing such selected fossil fuel burning equipment from the boiler as is necessary to complete the conversion;

b. removing and positioning as required those boiler elements necessary to expose the fire tube ends on both the front and rear tube sheets;

c. separating a selected plurality of fire tube ends from and where connected to said respective fire tube sheets and removingsuch selected tubes from said boiler whereby to provide corresponding mating pairs of openings in each said sheet;

d. between each said mating pair of openings installing an electrical immersion heater assembly which includes in the assembly respective plug means to hydraulically seal each respective opening, a support bar means extending between the respective openings and supported at each end internally of the boiler on the respective plug means, an elongated electrical immersion heater unit extending between said openings and supported on said bar means and power supply leads extending through one of said plug means and connected to said heater unit; and

e. connecting the plurality of said heater units provided in place of said selected fire tubes to appropriate electrical power supply and control means whereby said boiler is converted to heat and steam water therein upon electrical energization of said units through said power supply and control means.

2. The method of converting a fire tube boiler according to claim 1 and including the step of insulating the combustion chamber, selected breeching and other boiler areas normally exposed to the combustion process whereby to reduce the radiation losses induced by said electrical energization.

3. The method of converting a fire tube boiler according to claim 1 wherein selected said heater units are installed but left electrically disconnected and available as spares".

4 The method of converting a boiler according to claim 1 wherein in said step of connecting said heater units to appropriate electrical power supply and control means said heater units are connected by groups and are electrically supplied and controlled by groups.

5. A modified fossil fuel fired fire tube boiler converted to electrical operation, comprising in combination:

a. a shell structure having appended smoke boxes and related breeching, a combustion chamber and a water tank having front and rear fire tube sheets; b. a plurality of undisturbed fire tubes mounted in the conventional manner between said firetube sheets but being substantially less in number than the number of tubes normally mounted between said sheets; c. in place of selected removed fire tubes and between each mating pair of openings provided by each such removed fire tube, an electrical immersion heater assembly which includes in the as sembly respective plug means to hydraulically seal each respective opening, a support bar means extending between the'respective openings and supported at each end internally of the boiler on the respective plug means, an eloggated electrical immerslon heater umt exten mg between said said combustion chamber, selected breeching and other boiler areas normally exposed to the combustion process whereby to reduce the radiation losses induced by said electrical energization.

7. A modified fossil fuel fired fire tube boiler according to claim 5 having selected said heater units electrically disconnected and available as spares.

8. A modified fossil fuel fired fire tube boiler according to claim 5 wherein said electrical power supply and control means is adapted to electrically energize and deenergize said heater units by groups of heater units. 

1. The method of converting a fossil fuel fired fire tube boiler of the type having fire tubes extending between front and rear tube sheets, an associated combustion chamber, smoke boxes and fossil fuel burning equipment, comprising the steps: a. removing such selected fossil fuel burning equipment from the boiler as is necessary to complete the conversion; b. removing and positioning as required those boiler elements necessary to expose the fire tube ends on both the front and rear tube sheets; c. separating a selected plurality of fire tube ends from and where connected to said respective fire tube sheets and removing such selected tubes from said boiler whereby to provide corresponding mating pairs of openings in each said sheet; d. between each said mating pair of openings installing an electrical immersion heater assembly which includes in the assembly respective plug means to hydraulically seal each respective opening, a support bar means extending between the respective openings and supported at each end internally of the boiler on the respective plug means, an elongated electrical immersion heater unit extending between said openings and supported on said bar means and power supply leads extending through one of said plug means and connected to said heater unit; and e. connecting the plurality of said heater units provided in place of said selected fire tubes to appropriate electrical power supply and control means whereby said boiler is converted to heat and steam water therein upon electrical energization of said units through said power supply and control means.
 2. The method of converting a fire tube boiler according to claim 1 and including the step of insulating the combustion chamber, selected breeching and other boiler areas normally exposed to the combustion process whereby to reduce the radiation losses induced by said electrical energization.
 3. The method of converting a fire tube boiler according to claim 1 wherein selected said heater units are installed but left electrically disconnected and available as ''''spares''''.
 4. The method of converting a boiler accoRding to claim 1 wherein in said step of connecting said heater units to appropriate electrical power supply and control means said heater units are connected by groups and are electrically supplied and controlled by groups.
 5. A modified fossil fuel fired fire tube boiler converted to electrical operation, comprising in combination: a. a shell structure having appended smoke boxes and related breeching, a combustion chamber and a water tank having front and rear fire tube sheets; b. a plurality of undisturbed fire tubes mounted in the conventional manner between said fire tube sheets but being substantially less in number than the number of tubes normally mounted between said sheets; c. in place of selected removed fire tubes and between each mating pair of openings provided by each such removed fire tube, an electrical immersion heater assembly which includes in the assembly respective plug means to hydraulically seal each respective opening, a support bar means extending between the respective openings and supported at each end internally of the boiler on the respective plug means, an elongated electrical immersion heater unit extending between said openings and supported on said bar means and power supply leads extending through one of said plug means and connected to said heater units; and d. electrical power supply and control means connected to selected said heater units and adapted to electrically and selectively energize such units and heat and steam water in said tank according to predetermined temperature, pressure and water level demands.
 6. A modified fossil fuel fired fire tube boiler according to claim 5 and including insulation means applied to said combustion chamber, selected breeching and other boiler areas normally exposed to the combustion process whereby to reduce the radiation losses induced by said electrical energization.
 7. A modified fossil fuel fired fire tube boiler according to claim 5 having selected said heater units electrically disconnected and available as ''''spares''''.
 8. A modified fossil fuel fired fire tube boiler according to claim 5 wherein said electrical power supply and control means is adapted to electrically energize and deenergize said heater units by groups of heater units. 